A disk drive is a data storage device that stores digital data in tracks on the surface of a data storage disk. Data is read from or written to a track using a transducer that is held close to the track while the disk spins about its center at a substantially constant angular velocity. To properly locate the transducer near the desired track during a read or write operation, a closed-loop servo control system reads a servo pattern from the disk surface to generate a position error signal (PES) and align the transducer with the desired track.
The servo pattern typically includes short servo bursts of a constant frequency signal. The servo bursts are written in a servo sector on the track, are very precisely located and are offset from either side of the track centerline. The servo bursts can be used to find the track centerline. Keeping the transducer on-center is required during both reading and writing to and from the track. The servo bursts allow the transducer to follow the track centerline around the disk even when the track is perturbed (out-of-round) due to spindle wobble, disk slip and/or thermal expansion.
Servo bursts are conventionally written on the disk in the disk drive by a dedicated, external servo track writer (STW). The STW typically uses a large granite block to support the disk drive and reduce outside vibration. Unfortunately, the STW is expensive and requires a clean room since the disk and the transducer are exposed to allow access by the STW.
Disk drives have been developed that use self-servo writing (SSW) for writing the servo pattern. SSW typically uses a temporary pre-existing servo reference pattern on the disk to position the transducer while the servo bursts are written to the disk. SSW involves three largely distinct steps: (1) reading the reference pattern to provide precise timing, (2) positioning the transducer at a sequence of radial positions using amplitude variations in a readback signal from the reference pattern as a sensitive position indicator, and (3) writing the servo bursts at the times and radial positions defined by the first two steps to form concentric circular tracks. SSW is described in U.S. Pat. No. 5,907,447 to Yarmchuk et al. SSW can also involve servo-propagation where the servo reader-to-writer offset allows servoing on written servo bursts while writing other servo bursts.
In an ideal disk drive, the tracks are non-perturbed circles situated about the center of the disk. These ideal tracks include a track centerline that is located at a known constant radius from the disk center. In an actual system, however, it is difficult to write non-perturbed circular tracks to the disk. Vibration, bearing defects, etc. can result in tracks that are written differently from the ideal non-perturbed circular track shape. Positioning errors created by the perturbed nature of these tracks are known as written-in repeatable runout (WRRO).
The perturbed shape of these tracks complicates the transducer positioning during read and write operations performed after SSW because the servo control system needs to continuously reposition the transducer during track following to keep up with the constantly changing radius of the track centerline with respect to the center of the spinning disk. Furthermore, the perturbed shape of these tracks can result in track squeeze and track misregistration errors during read and write operations.
Disk drives have been developed that measure the WRRO for each track, generate compensation values (also known as embedded runout correction values or ERC values) and write the ERC values to servo sectors in the tracks. Thereafter, during read/write operations, the ERC values are used to position the transducer along an ideal track centerline. This is described in U.S. Pat. No. 6,549,362 to Melrose et al. (the '362 patent), which is incorporated herein by reference.
Although the ERC values correct or reduce the WRRO, generating the ERC values can be time consuming. After SSW, the WRRO on each track is measured and then the ERC values are calculated. Finally, the ERC values are written to each servo sector of each track. This requires several revolutions of the disk to measure the WRRO on the track and then more revolutions of the disk to write the ERC values to the track. In one example, this may require 12 or more revolutions of the disk to determine and write the ERC values for each track.
There is, therefore, a need for a disk drive that performs SSW and reduces the time required to provide ERC values.